The present invention relates to a process for the preparation of powder-form, heat-curing reaction mixtures by which solid polyisocyanate and high molecular weight isocyanate-reactive compounds that are solid at room temperature are added as melt suspensions to an inert solvent containing an emulsifier. The resultant powdered reaction mixture contains the starting components in uncrosslinked form.
German Offenlegungsschrift 2,330,601 (believed to correspond to U.S. Pat. No. 3,917,741) describes a process for the direct preparation of polyurethanes in finely divided form in which the polyol and polyisocyanate starting components are reacted in an inert solvent in the presence of a polymeric surface-active agent (emulsifier). The characteristic feature of this process is that the polyol compound, which is insoluble in the solvent, is finely emulsified by the emulsifier. A polyisocyanate, which is soluble in the particular solvent, is then added in liquid form to the resulting emulsion. A polyaddition reaction takes place at the phase boundary of the polyol droplet and the resultant polyurethane, which is also insoluble in the solvent, precipitates as a finely divided powder. The polyurethane is completely crosslinked and, when processed in molten form, shows good mechanical properties. The powders are used, for example, as paints or coatings and as adhesives for sheet-form textiles. Industrial articles are produced by press-molding.
Unfortunately, these powders are attended by the serious disadvantage of having their processing temperature very close to the temperature at which polyurethane recleavage reactions begin. Since these thermoplastic polyurethanes already have their final molecular weight, which must be relatively high if they are to show good mechanical properties, they have high melt viscosities at the maximum temperature at which they can be processed. Often, therefore, the flow behavior of the melt is also adversely affected to a considerable extent.
German Offenlegungsschrift 2,330,601 also indicates that both starting components (i.e., the NCO-reactive compound and polyisocyanate) may be insoluble in the inert solvent and that one of the starting components may even be present as a solid. In this case, it is also possible to add a suspension of the solid in the NCO-reactive compound to the inert solvent containing an emulsifier. No corresponding Examples are given in German Offenlegungsschrift 2,330,601.
An interfacial polyaddition reaction always takes place and fully reacted polyurethanes are obtained in finely divided form.
A continuous process involving interfacial polyaddition reactions is also disclosed in U.S. Pat. No. 4,940,750.
German Offenlegungsschrift 2,556,945 claims a process for the direct preparation of polyurethane powders in which the processing disadvantages mentioned in German Offenlegungsschrift 2,330,601 do not arise and the end products retain their favorable mechanical properties. In the disclosed process, low molecular weight and high molecular weight starting polyols are emulsified with an additional monofunctional isocyanate blocking agent (for example, caprolactam) in an inert solvent in the presence of a surface-active compound. After the addition of an aliphatic or aromatic polyisocyanate, a polyurethane powder that still contains free OH and NCO groups is formed. These powders show distinctly lower melt viscosity, coupled with a lower processing temperature, but this temperature (150.degree. C. to 190.degree. C.) is still sufficient to split the thermally unstable NCO adduct. The fully reacted polyurethane is finally obtained after reaction with the free OH groups of the NCO-reactive compound.
A disadvantage of this process is that the blocking agent is regenerated during the final curing phase and, in the case of caprolactam, for example, can sublime out or at least can accumulate at the surface of the molding ("exudation"). The latter behavior applies to many known NCO blocking agents.
The already distinctly reduced processing temperature of the resultant powder is in the range used in the processing of thermoplastics by standard methods and is accepted for such use. However, by comparison with the crosslinking reactions typical of polyurethane chemistry, this processing temperature is high and involves considerable energy consumption.
Accordingly, the problem addressed by the present invention was to provide a simple process for the preparation of storable polyurethane powders that can be cured at a temperature of only 100.degree. C. to 150.degree. C. and which, after heat curing, no longer contain any free NCO groups.
The present invention was also based on the concept of preparing polyurethane powders using solid polyisocyanates with an anti-diffusion layer at the particle surface, of the type obtained by the action of a small amount of aliphatic diamine on a solid diisocyanate, rather than using solid polyisocyanates in which the NCO groups are not blocked by thermally unstable adducts. Solid diisocyanates modified in this way are "deactivated" with respect to attack by NCO-reactive compounds. The anti-diffusion layer is destroyed or made permeable only by the action of heat or organic solvents or by the action of shear forces, thereby allowing the polyaddition reaction to take place. The process for "retarding solid polyisocyanates" is described in German Offenlegungsschrift 3,230,757 (European Patent Application 103,323 and believed to correspond to U.S. Pat. No. 4,483,974).
German Offenlegungsschrift 2,330,601 teaches that crosslinked polyurethane powders are also formed when one of the starting components is present as a solid. This solid may optionally be dispersed in the other, liquid component. If this heterogeneous mixture is added to the inert solvent in the presence of an emulsifier, fully reacted polyurethane powders are obtained after a certain reaction time.
Accordingly, several tests were carried out using methods known in the art using starting compounds not yet cited in the patent literature. An amine-terminated liquid polyether that is highly reactive to isocyanate groups (prepared in accordance with European Patent Application 219,035 by hydrolysis of an NCO preadduct of one mole of polypropylene glycol ether (molecular weight 2,000, OH value 56) and two moles of toluene diisocyanate ("TDI")) was used as the high molecular weight component. A solid diisocyanate (dimeric TDI, or "TT") provided with an anti-diffusion layer (see European Patent Application 103,323) was used as the polyisocyanate component. The following procedure was used according to the invention. The dimeric TDI ("TT") was added in an equivalent quantity to a solution containing a small quantity of an aliphatic diamine (i.e., isophoronediamine) in the NH.sub.2 -terminated polyether based on polypropylene glycol ether (molecular weight 2,000). The quantity of aliphatic diamine was selected to be just sufficient to form an anti-diffusion layer, as could be readily determined by one skilled in the art, within the range of 0.01-20 equivalent % of aliphatic amine based on solid isocyanate, preferably 0.1-3 equivalent %. A thin polyurea shell acting as an anti-diffusion layer formed on the surface of the solid TT particles after a short time, thereby forming a dispersion that was stable at room temperature. The deactivated material could be cured only by heating (100.degree. C. to 120.degree. C.), resulting in the formation of a solid highly elastic molding of high thermal stability. The described dispersion was then added to an inert solvent (i.e., hexane) in the presence of a surface-active compound (ANTARON V, a product of GAF-Europa), the mixture being effectively emulsified in hexane by means of a high-speed stirrer. An emulsion of finely divided droplets in the inert solvent was obtained at room temperature. After stirring, the two phases separated and the droplets changed into relatively coarse particles after standing for a prolonged period. After separation of the hexane, the reaction product used was recovered in unchanged form. No polyaddition had taken place.
This result is surprising in two respects and could not have been expected from the patent literature.
According to German Offenlegungsschrift 2,330,601, fully reacted polyurethanes are obtained when using an inert solvent containing emulsifiers, even when neither the polyol component nor the polyisocyanate component dissolves in the inert solvent and one of the starting components is present as a solid. This teaching, however, does not apply in the test described above, which is all the more surprising because the components used in the test are highly reactive components containing NH.sub.2 groups.
German Offenlegungsschrift 3,230,757, states that, although combinations of "retarded solid polyisocyanates" (those having an anti-diffusion layer formed by a polyurea shell) and a low molecular weight or high molecular weight NCO-reactive compound are stable in storage at room temperature, a spontaneous polyaddition reaction does take place under the effect of solvents (partial dissolving or swelling of the anti-diffusion layer) or under the effect of shear forces (high-speed stirrer). This teaching also does not apply in the test described above. Crosslinked polyurethane ureas are not obtained in either case and the starting components are recovered in unchanged form.
In further tests, the NH.sub.2 -containing polyether that is liquid at room temperature was replaced by an NH.sub.2 -containing polyester that is solid at room temperature (i.e., a polyadipate). Accordingly, mixing with the "stabilizing diamine" isophoronediamine ("IPDA") and the dimeric TDI ("TT") was carried out at a temperature above the melting temperature of the NH.sub.2 polyester (50.degree. C. to 60.degree. C.). Even at this temperature, no reaction between the components occurred because of the anti-diffusion layer produced by IPDA on the surface of the TT particles. The resultant melt was added dropwise with stirring to an inert solvent (i.e., hexane) that also contained a surface-active compound. A solid powder consisting of unchanged NH.sub.2 polyester and TT was obtained after a short time at room temperature. The powder, which accumulated as spherical particles, was free-flowing (particle size 5 to 200 .mu.m) and could be processed by standard "powder technology".
In contrast to the polyurethane powders described in German Offenlegungsschriften 2,330,601 and 2,556,945, the powders produced by the process according to the invention are uncrosslinked. These powders afford the crucial processing advantages of a relatively low starting viscosity of the melt and rapid setting to high-quality products at low curing temperatures.